1. Field of the Invention
This invention relates to fiber optic intelligence transmission, and more specifically to apparatus for encoding a position signal to be transmitted.
2. Description of the Prior Art
Modern industrial equipment has come to rely on sophisticated parameter controlling devices for optimization of operating characteristics. Critical to the operation of such parameter controlling devices is the precise position sensing of equipment subunits. For example, in modern gas turbine engines such controllable parameters include fuel flow, flowpath contour and pressure along the flowpath. Positionable units for controlling such parameters include fuel metering valves, moveable exhaust nozzles, and flowpath relief valves. In effective coordination of the variable parameters, it is essential that the actual position of the controlling units be continuously known.
The accuracy of the entire controlling system is, of course, largely dependent upon the accuracy of the position encoding devices which develop signals to be processed. Fiber optic systems are known to provide effective data transmission, particularly in hostile environments subject to electromagnetic interference and thermal fluctuations. One such hostile application is illustrated in U.S. Pat. No. 4,116,000 to Martin et al entitled "Engine Control System" which is of common assignee herewith. In the Martin et al patent the encoding device senses the position of the exhaust nozzle of a gas turbine engine. High temperatures at the exhaust nozzle and electromagnetic interference along the data transmission lines make fiber optic systems attractive for this application.
One inherent problem with fiber optic devices, however, is the packaging of the multiple channels required for encoding and transmitting bits of position information. A greater number of channels is desired for encoding more accurate position sensing. A lesser number of channels simplifies the encoder design and reduces the physical size of the encoder. One technique for reducing the number of channels without reducing the number of information bits is disclosed in U.S. Pat. No. 4,117,460 to Walworth et al entitled "Sensing Device", also of common assignee herewith. In the Walworth et al device the information signals are sequenced to provide multiple bits of information over each channel. The size of the transmission cable is reduced. The size of the encoding device, however, remains substantially unaltered. Both the Martin et al and Walworth et al patents illustrate flat plate encoders.
As more accurate sensing becomes desired, encoder size becomes an increasing problem. Each new channel, such as is added for accuracy, requires an additional aperture row on the encoder plate. An encoder plate of increased size results. Large encoder plates of a flat design not only tend to skew in their mounting tracks, but are also susceptible to thermal and vibratory distortions. Mechanical interferences impeding movement of the encoder plate may cause signal delays and hysteresis effects in the encoded signal. Significant clearance between transmitting and receiving heads of the encoder is provided in conventional systems to accommodate such anticipated distortions. Additionally, friction free mounting apparatus capable of providing precise alignment of the code plate tends to be large and bulky.
Manufacturers and designers of fiber optic encoders continue to search for new techniques that enable reductions in encoder size and for new packaging concepts which improve the mechanical operation of the code plate.